A:①②
B:②③
C:③④
D:②④
在原子核中,带正电的粒子是( )
As we grow up, it always _______ that after we give something up, we gain something new in its place. ( )
如果某地人口出生率为1.6%,死亡率为0.4%,其人口自然增长率为( )